National Speleological Society
Brush up on your caving basics and lingo (beginning with "spelunking"—the formal word for caving) at the website of the NSS, an organization devoted to the study, exploration, and conservation of cave and karst resources.
Speleogenesis and Evolution of Karst Aquifers Journal
Here's a searchable online scientific journal covering international studies on solution caves, hydrogeology, and the evolution of karst topography.
Bates, Robert L., and Julia A. Jackson, eds. Dictionary of Geological Terms, 3rd ed. American Geological Union, Anchor Press/Doubleday, 1984.
Klimchouk, Alexander. "Call of the Abyss." National Geographic (May 2005), 70-85.
Aulenbach, Nancy Holler, Hazel A. Barton, and Marfé Ferguson Delano. Exploring Caves: Journeys into the Earth. National Geographic Books, 2001.
Roberts, David. "Exploring the Place of Fright." National Geographic Adventure (July/August 2001), 52-60, 62, 64, 66, 130.
Taylor, Michael Ray. Caves: Exploring Hidden Realms. National Geographic Books, 2000.
Brower, Kenneth. "Face to Face in New Guinea." National Geographic Traveler (March 2000), 82-96.